Permanent magnet synchronous motors of this type are, for example, used in drive devices of motor vehicles, in particular in power-assisted steering systems and the like.
FIG. 1 schematically shows the structure and the mode of functioning of a generally known power-assisted steering system 100 for a motor vehicle. The power-assisted steering system shown there comprises a steering gear 101, a steering shaft 102 with a steering wheel, not designated in more detail, and a servo drive 103 with a control unit 104. A track rod 107 cooperates with the steering gear 101 by means of a pinion 105. The steering gear 101 has a gear rack (only indicated symbolically by a double arrow) here, which is connected to the pinion 105. The pinion 105 is rotatably coupled to the steering wheel by the steering shaft 102. The track rod 107 cooperates with steerable wheels 106 of the motor vehicle.
The servo drive 103 is equipped, for example, with a three-phase electric motor and is attached here with the control unit 104 between the steering wheel and the steering gear 101. The servo drive 103 is used as assistance during steering operations in that it cooperates with the gear rack. The servo drive 103 can be attached on a steering column for this purpose.
A motor currently used as a servo drive 103 may be a brushless motor, for example a permanent magnet synchronous motor or an asynchronous motor. For example, a permanent magnet synchronous motor has a stator with stator grooves, in which windings are arranged, and a rotor with permanent magnets. The permanent magnets are, for example, produced from a rare earth material and arranged in a spoke-like manner in the rotor.
A motor of this type has to satisfy at least the following two main requirements: 1) performance and 2) safety. In relation to the performance, parameters such as, for example, power density, maximum output torque, torque fluctuation or ripple and cogging torque or pulsating torque are important. In the case of safety, the parameter of maximum brake torque is, inter alia, critical at a phase short circuit and, for example, has to be limited to a value of about 0.6 Nm at a complete short circuit of a phase.
WO 2002/060740 A2 and EP 1028047 B1 provide examples or solution proposals for the above problems.
The increased requirements of the use conditions in a motor vehicle, in particular in relation to as small an installation volume as possible, low weight, low number of individual parts used and a simultaneously high efficiency, and the above-mentioned parameters result in the constantly present requirement of providing a correspondingly improved permanent magnet synchronous motor.